1. Technical Field
The present invention relates generally to laser ignition apparatuses for ignition of internal combustion engines. More particularly, the invention relates to a laser ignition apparatus for ignition of an internal combustion that is difficult to be ignited, such as a highly-charged engine, a high-compression engine or a natural gas engine that has large bore cylinders.
2. Description of Related Art
In recent years, various laser ignition apparatuses have been proposed for ignition of internal combustion engines that are difficult to be ignited; those engines include, for example, highly-charged engines, high-compression engines, and natural gas engines with large bore cylinders. The laser ignition 20 apparatuses are generally configured to: (1) irradiate an excitation light generated by an excitation light source (e.g., a flash lamp or a semiconductor laser) to a laser resonator (or optical resonator) that includes a laser medium and a Q switch, thereby causing the resonator to generate a pulsed laser light that has a short pulse width and a high power density; and (2) focusing the pulsed laser 25 light, using an optical element (e.g., a focusing lens), to a focal point (or an ignition point) in a combustion chamber of the engine to generate a flame kernel that has a high power density, thereby igniting the air-fuel mixture in the combustion chamber.
For example, a first prior art document (i.e., “Laser Ignition-a New Concept to Use and Increase the Potentials of Gas Engines” presented by Dr. Günther Herdin et al., ICEF2005-1352 (page 1-9), ASME Internal Combustion Engine Division 2005 Fall Technical Conference: ARES-ARICE Symposium on Gas Fired Reciprocating Engines, Sep. 11-14, 2005, Ottawa, Canada) discloses a laser ignition apparatus for ignition of a gas engine. The laser ignition apparatus includes a combustion chamber window. Further, when the power density of a laser light generated by the laser ignition apparatus is higher than or equal to a predetermined threshold, the apparatus can exert an effect of burning off contaminants (e.g., unburned fuel or soot) that has deposited on a combustion chamber-side end surface of the combustion chamber window; the predetermined threshold is close to the strength limit of the combustion chamber window.
A second prior art document (i.e., Japanese Unexamined Patent Application Publication No. 2010-116841) discloses a laser ignition apparatus which includes: a protective cover for protecting a focusing lens of the apparatus; means for detecting contaminants having adhered to a combustion chamber-side end surface of the protective cover; and means for burning off the contaminants with a laser light that has a predetermined power density.
However, in either of the laser ignition apparatuses disclosed in the first and second prior art documents, when the laser light with the predetermined power density is irradiated for burning off the contaminants having deposited on or adhered to the combustion chamber-side end surface of the protective cover (or the combustion chamber window), a pseudo mirror may be formed by the protective cover that is fouled with the contaminants. Consequently, part or the whole of the irradiated laser light may be reflected by the pseudo mirror, forming a catoptric-light focal point on the anti-combustion chamber side (i.e., the opposite side to the combustion chamber) of the protective cover; at the catoptric-light focal point, a catoptric light resulting from the reflection of the laser light by the pseudo mirror is focused.
Further, when the catoptric-light focal point is positioned within the focusing lens or the protective cover, concentration of the energy of the catoptric light may occur in the focusing lens or the protective cover, generating a plasma or a shock wave therein. Consequently, damage may be made to the focusing lens or the protective cover, such as causing cracks to occur in the focusing lens or the protective cover or causing an AR (Anti-Reflective) coating formed on the surface of the focusing lens to be peeled off.
Furthermore, due to the damage made to the focusing lens or the protective cover, scattering of the laser light may occur when it passes through the damaged part of the focusing lens or the protective cover, thereby lowering the power density of the laser light at the focal point in the combustion chamber. Consequently, it may become difficult for the laser ignition apparatus to reliably ignite the air-fuel mixture in the combustion chamber.